Abstract

Using subsurface drip irrigation (SDI) with lagoon wastewater has many potential advantages. The challenge is to design and manage the SDI system to prevent emitter clogging. A study was designed and initiated in 1998 to test the operation five types of driplines (with emitter flow rates of 0.57, 0.91, 1.5, 2.3, and 3.5 l/hr/emitter) with lagoon wastewater. Filtration was with a disk filter (200 mesh, with openings of 0.0030 inch) and shock treatments of chlorine and acid were injected periodically. During the 1998 growing season, 20.9 inches of wastewater was applied through the SDI system; 15.3 inches were applied in 1999. During the growing seasons, the two lowest flow rate emitter designs (0.15 and 0.24 gal/hr/emitter) decreased in flow rate, indicating that some emitter clogging had occurred. The magnitudes of the decreases were 15% and 11% of the original flow rates in 1998 and 21.5% and 13.7% in 1999 for the 0.15 gal/hr/emitter and 0.24 gal/hr/emitter driplines, respectively. After the winter idle period, the flow rates of both dripline types returned to the initial flow rates. The three highest flow rate emitter designs showed no signs of clogging; their flow rates decreased by 4% or less through both growing seasons. Observations showed that the disk filter and automatic backflush controller performed adequately in 1998 and 1999. Based on these results, the use of SDI with lagoon wastewater shows promise. However, the smaller emitter sizes may be risky for use with wastewater and the long-term (greater than two growing seasons) effects are untested.

Introduction

Using subsurface drip irrigation (SDI) with water from animal waste lagoons (hereinafter called wastewater) has many potential advantages. They include but are not limited to reduced human contact with wastewater; no runoff of wastewater into surface waters; placement of phosphorus-rich water beneath the soil surface where runoff potential is reduced; greater water application uniformity resulting in better control of the water, nutrients, and salts; reduced irrigation system corrosion; reduced climatic-based application constraint (especially high winds and low temperatures); and increased flexibility in matching field and irrigation system sizes.

Very small emitters in SDI systems may be prone to clogging by the various constituents of the wastewater. Emitter clogging is a major problem associated with SDI (Nakayama and Bucks, 1986). The design and management challenge of using SDI with wastewater is to prevent emitter clogging. Given that challenge, the objective of this project was to measure the performance of five different dripline types as affected by irrigation with filtered but untreated water from a beef feedlot runoff lagoon.

Methods

This project was conducted at a beef cattle feedlot in Gray County, Kansas. The soil type was a Richfield silt loam (USDA-SCS, 1968). Water collected in the lagoon was runoff from pens containing beef cattle. Selected wastewater characteristics are shown in Table 1.

In April 1998. driplines were installed 17 inches deep and on a lateral spacing of 5 ft. Each plot was 20 ft wide (4 driplines) and 450 ft long. The system installation and testing were completed on June 16. The first wastewater was used for irrigation on June 17. After completion and testing of the system, the lagoon wastewater was the only water applied with the SDI system; no clean water was used for irrigation, flushing, or dripline chemical treatment. Plots were arranged in a randomized complete block design with three replications. There was a border plot (using the 0.4 gal/hr/emitter laterals) at each of the north and south ends for a total of 17 plots. The crops irrigated in 1998 and 1999 were corn (Zea mays L.).

Five drip irrigation lateral line (dripline) types, each with a different emitter flow rate (and thus different emitter size), were tested (Table 2). The wide range of emitter flow rates (and sizes) was selected to determine the optimum emitter size that would be less prone to clogging when used with lagoon wastewater. Agricultural designs of SDI in the Great Plains with groundwater typically use lower flow rate emitters. The emitter flow rates and flow path dimensions were obtained from the manufacturers.

Table 2. Selected emitter characteristics for the driplines used in the study of SDI with lagoon wastewater, 1998-1999.

Emitter flow rate, gal/hr

Flow path dimensions, width by height by length, inches

Flow path area, inch2

Operating pressure, psi

0.15

*

*

8

0.24

0.0212 by 0.0297 by *

0.000663 **

8

0.40

0.028 by 0.032 by 0.787

0.000896

10

0.60

0.034 by 0.037 by 0.713

0.001258

10

0.92

0.052 by 0.052 by 0.610

0.002704

***

* These dimensions were not available from the manufacturer.** The flow path was not rectangular so the area is not the product of the flow path width multiplied by the height.*** The 0.92 gal/hr/emitter product had a pressure-compensating emitter. Inlet pressure was greater than 30 psi.

The wastewater was filtered with a plastic grooved-disk filter with the flow capacity based on the filter manufacturer recommendations (1168 inch2 for our maximum flow rate of 120 gal/min). The disks were selected to provide 200-mesh equivalent (openings of 0.0030 inch) filtration even though the manufacturer recommendations for all driplines were filtration of 140 mesh (openings of 0.0041 inch) or finer. A controller was used to automatically backflush the filter after every hour of operation or when the differential pressure across the filter reached 7psi. To help keep bacteria and algae from growing and accumulating in the driplines and to clean lines of existing organic materials, acid and chlorine were injected into the flow stream simultaneously at injection points about 3 ft apart. Acid was added at a rate to reduce the pH to approximately 6.3. Flushing (10 dripline volumes) to clean the lines and injections took place on the schedule shown in Table 3.

To test the system, irrigations of 0.25 to 0.4 inch were applied daily from June through early September 1998. During the growing season of 1999, occasional irrigations of 0.25 inches were applied in June and July and daily irrigations of 0.30 to 0.40 inch were applied from mid-July until early September. Each plot received the same application amount for a given day so the run times for plots varied according to their emitter flow rates and emitter spacings. The average seasonal application per plot was 20.9 inches (range: 20.6 to 20.9 inches) in 1998 and 15.3 inches (range: 15.3 to 15.6 inches) in 1999. The 1998 amount greatly exceeded the crop water requirements but allowed a more thorough test of the SDI system. Between the growing seasons, the system was used on October 6 and 7 and November 17, 1998 (DOY 279, 280, and 321) when the system flow rates were tested.

Emitter flow rates for entire plots were measured approximately weekly. Pressure gauges at the head and tail end of the plots were used to measure the pressure within the driplines. Totalizing flow meters measured the amount and rate of wastewater delivered to each plot. To test the flow rate of the driplines in an entire plot, the flow amount to each plot was measured and timed for approximately 30 minutes. Inlet and flushline pressures were recorded. To account for the variation due to minor fluctuation of pressures from test to test, the inlet pressure was normalized to the design pressure (Table 2) using the manufacturer's emitter exponent for that dripline type.

Of the five dripline types tested, the three higher-flow emitter sizes (0.4, 0.6, and 0.92 gal/hr/emitter) showed little sign of clogging (Fig. 1). Flow rates at the end of the test for those emitters were within 4% of the initial flow rates, indicating that very little emitter clogging and resultant decrease of flow rate had occurred. The absence of emitter clogging indicates that emitters of these sizes may be adequate for use with lagoon wastewater.

The two lower-flow emitter sizes (0.15 and 0.24 gal/hr/emitter) showed some signs of emitter clogging (Fig. 1) during the 1998 and 1999 growing seasons. Within 30 days of system completion in 1998, the flow rates of plots with both smaller emitter flow rates began to decrease. The 0.15 gal/hr/emitter plots showed gradual decrease of flow rate throughout the remainder of the test. By November 17, 1998 (DOY 321), the flow rate had decreased by 15% of the initial flow rate. The 0.24 gal/hr/emitter plots showed a decrease of flow rate of 11% of the initial flow rate by September 2, 1998 (DOY 245). Following harvest and the first (32-day) idle period, the 0.24 gal/hr/emitter plot flow rates increased approximately 5% over the minimum measured flow rate. This increase indicates that some cleaning of the emitters had occurred in response to the flushing. The flow rate then stabilized for the rest of 1998 at about 9% less than the initial flow rate.

Following the winter idle period, all flow rates recovered to the initial flow rates (Fig. 1). Possible explanations for this include (a) the longer time for the acid and chlorine in the driplines allowed better control of biological clogging agents or (b) the cooler temperatures during the winter resulted in partial control of the biological clogging agents and the acid and chlorine were then more effective at cleaning up the remaining agents.

The smallest emitter sizes again showed decreasing flow rates during the 1999 growing season (Fig. 1), similar to the response in 1998. By the end of the 1999 growing season (September 10, 1999), the 0.15 gal/hr/emitter plots had decreased by 21.5% of the and the 0.24 gal/hr/emitter plots had decreased by 13.7% of the initial (maximum) flow rate. The decrease of flow rate was constant through the growing season. The rate of decrease was reduced during a period from August 10 to August 24, 1999. During the second week of that time period, no daily irrigations were applied.

The disk filter and automated backflush controller operated well in 1998 and 1999. Based on our observations, the hourly backflush frequency was adequate to prevent excessive differential pressure accumulation and the set point of 7 psi was never reached.

Excavation of dripline samples and visual inspection of those samples showed that flushing was effective in removing the larger accumulations of materials from the driplines. Prior to flushing, a slimy substance probably containing both silt and biological materials was present in the lines. After flushing, the driplines were clean.

These results show that the drip irrigation laterals used with SDI have potential for use with lagoon wastewater. It appears that the smaller emitter sizes normally used with groundwater sources in western Kansas may be risky for use with lagoon wastewater, even though the flow rates returned to the initial rates after the winter off-season.

The dripline performance was similar during two growing seasons but questions still remain about the long term, multi-season performance of SDI systems using livestock wastewater. These concerns are especially important in light of the decrease of flow rates of the two smallest emitters during both growing seasons. Long-term performance will probably be necessary to justify the high investment costs of SDI systems.

Acknowledgements

We thank Midwest Feeders for providing the site, wastewater, and assistance with the project. We also thank the numerous companies that donated irrigation products and services in support of this project. Funding for the project was recommended by the Governor's office, approved by the Kansas legislature in 1998, and administered through KCARE at Kansas State University.